Recently, various devices that require batteries, such as cellular phones, wireless home appliances, and electric vehicles, are being developed. As these devices are developed, the demand for secondary batteries is also increasing. Particularly, along with the tendency of miniaturization of electronic products, secondary batteries are also becoming lighter and smaller.
In accordance with this trend, lithium secondary batteries, which use lithium metal as active material, are attracting attention. Lithium metal has a low redox potential (−3.045 V vs. standard hydrogen electrode) and a high weight energy density (3,860 mAhg−1), and thus is expected to be a negative electrode material for high capacity secondary batteries.
The metal chalcogenide compound capable of intercalating and deintercalating lithium ions has been used as a positive electrode active material for this lithium secondary battery since the 1970s. However, due to the low operating voltage, composite metal oxides such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-yCoyO2 (0<y<1), and LiMnO2 have been replaced and put to practical use or studied.
In particular, LiCoO2 was reported to be useful as a positive electrode active material for lithium secondary batteries in 1980, and thereafter has been extensively studied so far and has been adopted as a positive electrode active material for commercial lithium secondary batteries. However, in a situation where the unit cost of preparation of LiCoO2 accounts for a substantial portion (about 25%) of the unit cost of preparation of the battery, and due to the increasingly accelerating competition in the field of lithium secondary batteries, makers of lithium secondary batteries are forced to do their best to further lower production costs.
The high unit cost of LiCoO2 is attributed to the following two reasons. First, the high unit cost of preparation of cobalt raw material is one of the reasons. Second, the costs for quality control and process control at mass manufacturing scale are another reason. Particularly, in terms of quality control and process control, makers of batteries aim to obtain highly reproducible products with optimized physical properties wherein all batches aim to have little variation from the above optimized performance. LiCoO2 with its high reproducibility and very small fluctuation of performance is an essential element in the production line for the mass production scale of lithium batteries, which is now considerably automated.